Why is it useful?

Twitter is still in its infancy. It is relevant, 'timely' conversation that is mostly public. You can listen to (follow) anyone you'd like to, and people can find you based on what you have in common - or if they see you to be saying interesting things.

Messages (tweets) from those you follow are organized in a feed called the timeline, which is a list with the most recent updates first. Scrolling down takes you back through the history of their tweets over time.

Why Not?

It's not a question of 'why?', but 'why not?'. Soon, a majority of people in the US will access the Internet using a smartphone. More and more this is being done near your place of business. While regular websites may eventually be viewable on all devices, there is no reason to limit your audience.